Aircraft: HELICOPTER
Phase of flight: Takeoff / Launch
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway

We contacted VNY ground to leave the ramp and taxi to 16L for a southwest departure. Ground cleared us to 'taxi via Taxiway B and D for 16L cleared for takeoff'. We held short at D and had to ask for frequency change to Tower. Ground advised 'contact Tower 119.0'. I contacted Tower with request for takeoff 16L for a southwest departure. Tower said '16L cleared for takeoff'. I repeated back to Tower what I had heard with no response. We continued out onto 16L. As we began takeoff Tower came on the radio advising 'Cessna hold short helicopter is entering runway hold short' then quickly advised 'helicopter entering runway the clearance was not for you it was for the Cessna holding short at Taxiway C'. Tower then advised us to call the Tower after arrival and shutdown. After departing the airspace copilot had told me that a similar situation had occurred the day prior with a different Captain where ground had cleared them for takeoff on 16L. This had not been an issue due to no other aircraft involvement in the situation. We contacted tower after we had returned; they took down my information and said I would be contacted if there was any other questions. Later we were departing again from VNY for relocation of the aircraft. We contacted ground again and asked to taxi for departure. Ground came back over the ground frequency and said 'taxi to 16L via taxi way B; 16L cleared for takeoff' we repeated back and then proceeded to D and held short. Ground control came back and said '16L cleared for takeoff' we said to ground 'confirm 16L cleared for takeoff'? Ground replied with 'yes 16L cleared for takeoff'. We then replied 'we are still on ground do you want us to change frequencies to Tower'? Ground control said 'yes switch to Tower 119.0'. We switched to 119.0 and confirmed that we were still cleared for takeoff. Tower replied with '16L cleared for takeoff make left downwind departure to leave the area'.What has caused the most confusion for me and my copilot are.1. Ground clearance had been saying '16L cleared for takeoff'2. Copilot had a problem due to Tower not keeping a consistent Tower frequency switching from fixed wing frequencies and rotor wing frequencies. 3. The last time I had been to the area we were able to depart direct from our location with a large aircraft and now with large aircraft we are required to ground taxi from the east side of the airport. Helicopter mistook a takeoff departure clearance for a Cessna and departed Runway 16 Left.I was working CIC and halfway monitoring the local control positions. The Ground controller taxied Helicopter to Runway 16 Left (intersection Delta) as our procedures dictate. I and the Ground Controller were talking about this helicopter and how big it was and how its roterwash is dangerous to especially small aircraft; etc. etc.I hear the Local controller cross Runway 16 Right with a small Cessna to hold short of Runway 16 Left. My personal thought was why is the Local intentionally putting a C172 that close to a large Helicopter (approx. 900 FT). I heard the Local controller clear someone for takeoff and 'a' readback. There was some overlap with another aircraft on frequency. I'm watching this unfold as the Helicopter takes to Runway 16 Left at delta and depart. The Local Controller starts complaining about 'what is this guy doing'. The Ground and Helo Positions reached out to the [helicopter] with no response. This is when it occured to me that the readback was from Helicopter and not the Cessna who had not moved from the hold short position. I shouted to the Local controller that the Helicopter was on his frequency. The Local Controller called [the helicopter] and the pilot responded. The Local Controller then rudely chastised the pilot.A point that I would like to make now is the poor response of the Local Controller to the situation. Generally we assume the helicopters are ready to depart as soon as they start their 300 ft taxi clearance to the runway. I confirmed after that 'YES' the Local saw the helo at the runway. The Local was attempting to depart the Cessna before the Helo on Runway 16 Left. It was slow motion and as Helo started to taxi onto Runway 16 Left it could have been stopped with a minor runway incursion. I saw that Cessna (that was cleared for Takeoff) wasn't moving. Also the local controller complained (off Freq) about this pilot. This situation with better listening and observations could have prevented it.Let the Helicopter Position work the Helicopters. The Helo Position can coordinate the use of the runway if needed.Keep procedures as simple as possible and normal as to what the pilots would expect. Helicopter pilot and two Controllers reported a runway incursion at VNY airport. Communication issues were cited as contributing factors.